To assess the timing of petroleum generation and predict the quantity and quality of petroleum fluids subsurface are pivotal in petroleum exploration. Petroleum fluid is generated from kerogen, which by definition is the fraction of organic matter in sedimentary rocks that is insoluble in usual organic solvents. Kerogen is a complex mixture of macromolecular materials, whose composition and structure evolve over geological time under the influence of burial temperature and pressure.
With the demise of living matter, such as diatoms, planktons, spores and pollens, organic matter begins to undergo decomposition or degradation. In this break-down process, large biopolymers from proteins and carbohydrates begin to dismantle either partially or completely. These dismantled components are units that can then polycondense to form polymers. This polymerization usually happens alongside the formation of a mineral component (geopolymer) resulting in a sedimentary rock, such as kerogen shale. The formation of polymers in this way accounts for the large molecular weights and diverse chemical compositions associated with kerogen. The smallest units are the fulvic acids, the medium units are the humic, and the largest units are the humins. See FIG. 1A-D.
When organic matter is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provides significant pressure and a temperature gradient. When these humic precursors are subjected to sufficient geothermal pressures for sufficient geologic time, they begin to undergo certain specific changes to become kerogen. Such changes are indicative of the maturity stage of a particular kerogen. These changes include loss of hydrogen, oxygen, nitrogen, and sulfur, which lead to loss of other functional groups that further promote isomerization and aromatization which are associated with increasing depth or burial. Aromatization then allows for neat molecular stacking in sheets, which in turn increases molecular density and vitrinite reflectance properties, as well as changes in spore coloration, characteristically from yellow to orange to brown to black with increasing depth.
As kerogen is a mixture of organic material, rather than a specific chemical, it cannot be given a chemical formula. Indeed its chemical composition can vary quite distinctively from sample to sample. Thus, kerogen is typed according to average content.
Type I: Sapropelic.
Type 1 kerogen oil shales yield larger amount of volatile or extractable compounds than other types upon pyrolysis. Hence, from the theoretical view, Type 1 kerogen oil shales provide the highest yield of oil and are the most promising deposits in terms of conventional oil retorting, containing alginite, amorphous organic matter, cyanobacteria, freshwater algae, and land plant resins. Typical features include:                Hydrogen:carbon ratio>1.25        Oxygen:carbon ratio<0.15        Shows great tendency to readily produce liquid hydrocarbons        Derives principally from lacustrine algae and forms only in anoxic lakes and several other unusual marine environments        Has few cyclic or aromatic structures        Formed mainly from proteins and lipids        
Type II: Planktonic:
Type II kerogen is common in many oil shale deposits. It is based on marine organic materials, which are formed in reducing environments. Sulfur is found in substantial amounts in the associated bitumen and is generally higher than the sulfur content of Type I or III kerogens. Although pyrolysis of Type II kerogen yields less oil than Type I, the amount acquired is still sufficient to consider Type II bearing rocks as potential oil sources. Typical features of Type II kerogen include:                Plankton (marine)        Hydrogen:carbon ratio<1.25        Oxygen:carbon ratio 0.03 to 0.18        Tend to produce a mix of gas and oil.        Great tendencies to produce petroleum and are all formed from lipids deposited under reducing conditions.        Several types:                    Sporinite: formed from the casings of pollen and spores            Cutinite: formed from terrestrial plant cuticle            Resinite: formed from terrestrial plant resins and animal decomposition resins            Liptinite: formed from terrestrial plant lipids (hydrophobic molecules that are soluble in organic solvents) and marine algae                        
Type II: Sulfurous:
Similar to Type II but high in sulfur.
Type III: Humic:
Kerogen Type III is formed from terrestrial plant matter that is lacking in lipids or waxy matter. It forms from cellulose, the carbohydrate polymer that forms the rigid structure of terrestrial plants, lignin, a non-carbohydrate polymer formed from phenyl-propane units that binds the strings of cellulose together, and terpenes and phenolic compounds in the plant. Type III kerogen involving rocks are found to be the least productive upon pyrolysis and probably the least favorable deposits for oil generation. Type III kerogen features include:                Land plants (coastal)        Hydrogen:carbon ratio<1        Oxygen:carbon ratio 0.03 to 0.3        Material is thick, resembling wood or coal        Tends to produce coal and gas, although recent research has shown that type III kerogens can actually produce oil under extreme conditions        Has very low hydrogen content because of the extensive ring and aromatic systems        
Type IV: Residue:
Type IV kerogen contains mostly decomposed organic matter in the form of polycyclic aromatic hydrocarbons. They have no potential to produce hydrocarbons. Features include a hydrogen to carbon ratio of <0.5.
As part of the evolution of kerogen, petroleum fluid is generated, a process referred as primary cracking. Also under the influence of burial temperature and pressure, the generated petroleum fluid itself evolves to increasingly lighter fluid via a series of reactions, a process referred as secondary cracking.
As any chemical reaction, the primary cracking and secondary cracking proceed at finite rates governed by reaction kinetics. The practice to derive the parameters that describe the kinetics of petroleum generation is generally referred as “source rock kinetics analysis” or “kerogen kinetics analysis.” Once derived correctly, kinetics is applied in geological settings to predict petroleum generation, as well as its alteration, quantity and quality.
Over the past decades, significant efforts have been dedicated to developing methods that are suitable to derive the kinetics of petroleum generation and alteration of generated petroleum, e.g. changing from black oil to volatile oil, in either petroleum source rock or the reservoir. Catering for different business needs, a few methods are available. The most widely used method is the bulk kinetics analysis based on programmed open system pyrolysis.
In bulk kinetics analysis, source rock or kerogen isolate sample is pyrolyzed at certain heating rate under an inert gas (e.g. helium or nitrogen) purge, which transfers the pyrolysis products to a FID for continuous measuring of hydrocarbons generated as pyrolysis proceeds. After performing this experiment by using a few different heating rates (typically from 0.1° C./min to 20° C./min), the bulk hydrocarbon generation kinetic parameters can be derived based on the measured hydrocarbon generation curves at different heating rates. This method is relatively cheap and fast, but only provides kinetic parameters for the overall transformation of kerogen to petroleum fluid, not compositional kinetics. Due to its open system nature, the pyrolysis products do not closely represent hydrocarbons generated subsurface.
To derive compositional kinetics based on bulk kinetics analysis, another technique, named MicroScale Sealed Vessel (MSSV) pyrolysis has been developed. In MSSV a number of small quartz vials, each of which is sealed with known amount of kerogen sample, are pyrolyzed at selected heating rates to selected end temperatures. Upon thermolysis, each vial is cracked open in a GC sampler and the products are analyzed directly by GC. Based on the product compositions of a series of MSSV experiments, a compositional kinetics model is derived from bulk kinetics by subdividing activation energy (Ea) with respect to its contribution to the generation of individual components. Strictly speaking MSSV approach is only semi-compositional, since it can only analyze products detectable by GC, leaving out heavier products. Also, it has limited ability to tackle secondary cracking.
Gold tube thermolysis is a more sophisticated compositional kinetics analysis method, in which kerogen or whole rock sample is sealed into a gold tube under inert atmosphere, and the sealed gold tubes are thermolyzed while being subjected to a confining pressure (to mimic subsurface conditions). After thermolysis of a series of tubes over a range of thermal stresses, detailed analyses are performed for gas, liquid and solid products generated in each tube. Based on the product composition changes over a range of thermal stresses (different combinations of temperature and time), a compositional kinetics model is derived via numerical regression/optimization of the experimental data. This numerical analysis process involves designing a reaction network, which describes the chemical changes and deriving the kinetics parameters for the reaction network.
During Rock-Eval analysis, whole rock or kerogen isolate sample is pyrolyzed using a programmed heating while being purged by an inert gas, e.g. helium or nitrogen, which carries the pyrolysis products to the detector. The pyrolysis products are carried by the purge gas to detectors. A flame ionization detector (FID) detects hydrocarbons released during each stage of heating. Infrared (IR) detector measures CO and CO2 released during pyrolysis and oxidation. A thermocouple monitors temperatures, and these measurements are recorded on a chart known as a pyrogram (see FIG. 2).
An exemplary pyrolysis oven temperature program is as follows: for 3 min, the oven is kept isothermally at 300° C. and the free hydrocarbons are volatilized and measured as the S1 peak (detected by FID). The temperature is then increased from 300° to 550° C. (at 25° C./min). This is the phase of volatilization of the very heavy hydrocarbons compounds (>C40) as well as the cracking of nonvolatile organic matter. The hydrocarbons released from this thermal cracking are measured as the S2 peak (by FID). The temperature at which S2 reaches its maximum depends on the nature and maturity of the kerogen and is called Tmax. The CO2 released from kerogen during pyrolysis in the 300°-390° C. temperature range is cold trapped first, then released warming up the cold trap and detected on a TCD (S3 peak).
In summary, the four key parameters obtained by Rock Eval are as follows:
S1=the amount of free hydrocarbons (gas and oil) in the sample (in milligrams of hydrocarbon per gram of rock).
S2=the amount of hydrocarbons generated through thermal cracking of kerogen and nonvolatile organic matter. S2 is the indication of generative potential and used to calculate hydrogen index (HI).
S3=the amount of CO2 (in milligrams CO2 per gram of rock) produced during pyrolysis of kerogen. S3 is an indication of the amount of oxygen in the kerogen and is used to calculate the oxygen index. Contamination of the samples should be suspected if abnormally high S3 values are obtained. High concentrations of carbonates that break down at lower temperatures than 390° C. will also cause higher S3 values than expected.Tmax=the temperature at which the S2 signal peaks. Tmax is an indication of the maturity.
The RE II apparatus can also be used to determine the total organic carbon or “TOC” of the sample by oxidizing (in an oxidation oven kept at 600° C.) the organic matter remaining in the sample after pyrolysis (residual organic carbon). The TOC is then determined by adding the residual organic carbon detected to the pyrolyzed organic carbon, which in turn is measured from the hydrocarbon compounds issuing from pyrolysis.
Currently used bulk kinetics and MSSV based compositional kinetics are inadequate for advanced fluids quality and property predictions. Gold tube thermolysis generates products better matching subsurface fluids, but compositional kinetics analysis based on gold tube thermolysis is too time-consuming and also prone to error. Thus, what is needed in the art is a better method of quickly and efficiently determining the compositional kinetics of hydrocarbon generation from kerogen and subsequent alterations of generated petroleum fluids.